Most U.S. automobile manufacturers have prepared specification manuals that include general requirements for the speedometer cable and speed sensor assembly that provides speed information to the speedometer, the odometer and also to other instrumentation requiring a speed signal such as onboard computers that provide visual indications of miles per gallon and time to destination. The speedometer cable includes a rotating flexible metal shaft covered by a nylon or polypropylene sheath or casing. This cable is drivingly connected to one of the available gears in the automobile transmission usually through a bore formed in an extension in the transmission's housing. The cable is connected to the housing using an enlarged cylindrical metal ferrule that fits closely in this housing bore and has an axial central passage that receives and grips the cable. The end of the cable projecting within the transmission housing, or more particularly the drive shaft which usually has a square configuration at the point where it projects from the case or sheathing, is attached to a sensing gear, sometimes a plastic molded pinion gear that interengages with a worm gear in the transmission. The transmission end of the speedometer cable is held in position in the housing by a clamping fork that holds a flange on the outside of the ferrule in its seated position against the transmission housing extension.
In the past, this mounting ferrule has been constructed of metal and while prior metal ferrules have been found satisfactory, the configuration of the ferrule and its size require several machining and forming operations that make the ferrule a significant contributor to the cost of the entire speedometer sensor and cable assembly. As may be appreciated, the competition to supply items such as speedometer cable assemblies to automobile manufacturers is extremely competitive, and thus the search for unique and ingenious ways to reduce the cost of such items without sacrificing the demands for high performance and reliability is indeed an intensive one.
One apparent way to reduce the cost of manufacturing and assembling the ferrule is to construct it of molded plastic, but thus far attempts at manufacturing plastic ferrules has not been satisfactory because of the rather severe enviroment to which the ferrule is subjected. Transmission temperatures frequently exceed several hundred degrees fahrenheit and this heat along with the heated transmission oil to which the ferrule is directly exposed because of its location in the housing, attack the plastic ferrule materials resulting in ferrule deformation that produces sensor gear misalignment as well as a weakening of the attachment of the ferrule to the cable casing.
One prior attempt to design a plastic ferrule includes a plurality of axially extending flexible relatively thin fins on the ferrule that are press fitted and deformed slightly during assembly into the transmission housing bore in an effort to minimize the alignment problems and at the same time provide a lower cost ferrule. However it has been found that these flexible axial fins or ribs distort under the severe conditions in the transmission housing causing a misalignment of the sensor gear connected end of the speedometer shaft which cannot be tolerated because excellent meshing engagement must be maintained between the sensor gear and the engaged transmission gear. And this is particularly critical in installations where the sensor gear is installations where the sensor gear is cantilevered in the transmission housing. A plastic ferrule of this type is shown and described in the Bartholomew U.S. Pat. No. 3,610,056.
It is the primary object of the present invention to ameliorate the problems noted above in speedometer cable assemblies.